1. Field of the Invention
The present invention pertains to the field of frequency control and in particular to an improved method of enclosing and supplying power to a crystal oscillator.
2. Description of the Prior Art
Crystal oscillators are used in controlling the frequencies of nearly all electronic devices. The main frequency and frequency stability determining element in crystal oscillators is the crystal resonator. It is known that a crystal resonator's resonance frequencies change with temperature. In the highest stability crystal oscillators, the resonator temperature is kept constant in an oven. The principal drawback of such oven controlled crystal oscillators (OCXO) is the power requirement, which usually prevents the use of OCXOs in battery operated equipment. The lower the operating temperature of the OCXO the more power that is required to maintain a constant oven temperature. Power requirements of commercially available OCXOs are typically in the range of 1 to 6 watts. Designers of OCXOs strive to minimize the power requirements by using low thermal conductivity materials, such as foam insulators, to isolate the oven from the environment and by minimizing heat losses through electrical leads. For battery operated equipment, the desired input power requirement is less than 100 milliwatts. See, for example, Frerking, Marvin E., "Crystal Oscillator Design and Temperature Compensation", Van Nostrand Reinhold Company, New York for a brief description of oven controlled crystal oscillators.
The best thermal insulator is a vacuum, together with appropriately finished surfaces in the oven to minimize heat losses by means of radiation. The tactical miniature crystal oscillator (TMXO), for example, uses a vacuum for thermal insulation, however, using a vacuum results in some undesirable side effects. The major side effects are resonances in the mounting structure and high manufacturing costs due to the complexity of assembly. Part of the complexity results from the necessity for electrical leads which must extend from the components in the oven, through the thermal insulation to a connector on the outside of the oscillator enclosure. Supporting an oscillator in a vacuum insulated enclosure requires difficult tradeoffs between minimizing heat losses through the supporting structure and electrical leads and providing a sufficiently rigid structure for the vibration resonances to be inconsequential. Complex arrangements of wires and thin-walled tubes have been used in the TMXO in attempts to strike a balance between the conflicting requirements. Prior art attempts have been only partially successful. In this regard, see Symonds, D.A, et al "An Update on the TMXO", Forty-Fourth Annual Symposium on Frequency Control," 1990, IEEE; and Brown, D., et al "Manufacturing Methods and Technology For Tactical Miniature Crystal Oscillators, 38th Annual Frequency Control Symposium," 1984, IEEE.
Ideally, it would be desired to completely isolate an oscillator from external mechanical and thermal effects such as by suspending the oscillator in an ultrahigh vacuum in some fashion. Such a complete isolation is not possible on earth where one must apply forces to the oscillator to counteract the force of gravity. In addition, in the prior art, electrical leads were always used to supply power to the oven and oscillator circuitry, and for extracting the signal from the oscillator. It is an object of the invention to provide an improved method of supplying power to a crystal oscillator by transmitting power to the oscillator without wires.